1. Field of the Invention
The present invention relates to a rotational electric machine having a field winding formed by coupling formed coils to respective bobbins. The field winding of this type is advantageously used in a starter motor for cranking an internal combustion engine.
2. Description of Related Art
An example of a direct current motor having a field winding is disclosed in JP-A-2006-271121. In this motor, the field winding is formed by winding a flat wire to improve a space factor of the field winding and to thereby make the motor compact. The flat wire is a wire having a substantially rectangular flat cross-section, and the flat wire is edgewise-wound (wound by bending the flat wire in its width direction) in this motor. JP-B2-3737461 discloses a flat wire coil edgewise-wound around a bobbin. By using the flat wire, the space factor of the winding is improved, compared with a round wire.
There is a problem, however, in winding the flat wire directly around a bobbin, especially when winding it edgewise. To wind the flat wire edgewise, a high bending force has to be applied to the flat wire against its high rigidity. Accordingly, a mechanical strength of a bobbin has to be made high by increasing its wall thickness. This makes the bobbin size large. Accordingly, a length of the coil becomes longer and its electrical resistance higher, adversely affecting an output of the motor. To overcome the above problem, it may be conceived to couple a wound coil to a bobbin. However, it is difficult to hold the wound coil between the flanges because the bobbin has flanges which are larger than an inner hole of the wound coil.